1. Field of the Invention
This invention relates to an optical fiber and a device using the optical fiber, specifically to an optical fiber doped with a rare earth element and an optical fiber-type light amplifier (optical fiber amplifier).
2. Related Background Art
The light amplifier using an optical fiber doped with a rare earth element, e.g., erbium (Er) is characterized by little noise, and data transfer at high bit rates. Countries all over the world are studying and developing the optical fiber amplifier. Many results of these studies were presented in 1989 Optical Fiber conference held in Houston, U.S.A. in February, 1989. For example, Y. Kimura et al. presented "Efficient Er.sup.+3 -doped optical fiber amplifier pumped by a 1.48-.mu.m high-power laser diode" which proved that a laser diode is used as a beam source for exciting (pumping) the Er doped in a quartz fiber, whereby the optical fiber amplifier can be miniaturized.
It is necessary that the optical fiber amplifier comprises in a package a pumping beam source, an Er-doped optical fiber, and an optical coupler for combining a signal beam and a pumping beam. The optical coupler is generally small-sized, and the pumping beam source can be miniaturized by using a laser diode. But the Er-doped optical fiber has to be, at the present, as long as 50-200 m, and it is difficult to house this optical fiber in a small package. This is because when the optical fiber is wound in loops to be compact, the optical fiber more tends to break off as the radius of curvature becomes smaller. When the optical fiber is wound in a number of loops, the loops of the Er-doped optical fiber are adjacent to one another, with the result that the optical fiber becomes unsuitable for high bit-rate transmission. The stimulated emission light of the Er ions excited in the optical fiber is given forth not only in the axial direction of the optical fiber but also in all directions and intrudes into the adjacent loops of the otpical fibers as leakage radiation. Resultantly the leakage radiation as well is amplified unsuitably for high bit-rate signal transfer.
This problem occurs not only with the optical fiber amplifier but also with the optical fiber laser oscillator. That is, to miniaturize the laser oscillator, needless to say it is necessary to wind the Er-doped optical fiber into small loops. In other words, to miniaturize the optical fiber-type optically active device, such as a light amplifier, a laser oscillator or others, it is necessary to prevent breakage of loops of the optical fiber and shut off leakage light.
A first object of this invention is to provide an optical fiber which is bendable at a small radius of curvature and can prevent the leakage of the stimulated emission light.
A second object of this invention is to provide an optically active device, such as an optical fiber amplifier, which is small-sized and can realize good optically active characteristics.